Optical waveguide structure including reflective asymmetric cavity

ABSTRACT

A turning mirror in an optical waveguide structure is made by etching in the upper surface of the structure a cavity (18) that intercepts the path of light propagated by the waveguide (15, 16, 13). Preferably, the cavity is made to be asymmetric with the side (25) of the cavity remote from the waveguide sloping at typically a forty-five degree angle. The asymmetry can be introduced by using mask and etch techniques and treating the surface of the structure such that the etchant undercuts the mask on the side of the cavity remote from the waveguide to a greater extent than it undercuts the mask on the side of the cavity adjacent the waveguide.

This is a division of application Ser. No. 07/692,433 filed Apr. 29,1991, pending.

TECHNICAL FIELD

This invention relates to methods for making mirrors and, moreparticularly, to methods for making turning mirrors in conjunction withglass optical waveguides.

BACKGROUND OF THE INVENTION

One of the major advances in communications in recent years has been theincreased use of optical systems for carrying very large quantities ofinformation with low distortion and at low cost over great distances.Optical systems are also promising for such purposes as computingbecause of the inherently high speeds at which they can be operated. Forthese reasons, considerable work has been done to develop convenienttechniques for operating on transmitted information-carrying light toproduce various device functions. Devices known variously as hybridoptical integrated circuits, photonics modules, or hybrid opticalpackages have been proposed for controlling light using waveguidepatterns similar to the electronic circuit patterns used in electronicintegrated circuits.

The paper, "Glass Waveguides on Silicon for Hybrid Optical Packaging,"C. H. Henry et al., Journal of Lightwave Technology, Vol. 7, No. 10,October 1989, pp. 1530-1539 describes a method using successive layersof glass over a silicon substrate to define optical waveguides. Onelayer of glass having a relatively high refractive index is the corelayer and is surrounded by glass having a lower index of refraction.During operation, the light is confined in the core glass because of thelower refractive index of surrounding glass and, as a consequence, theconfiguration of the core layer defines the path of the light. Suchwaveguide configurations, sometimes referred to as optical circuits, canbe fabricated with precision by masking and etching of the core layer.The Henry et al. paper describes how various passive devices such ascouplers and polarization splitters can be made from optical waveguidesfabricated in this manner. A similar approach for defining opticalcircuits is described in the patent of Kawachi et al. U.S. Pat. No.4,557,099, granted Jun. 14, 1988.

Any hybrid optical packaging approach of the type described above willtypically require a number of forty-five degree mirrors, known in theart as turning mirrors, for coupling light between optical waveguides ofthe device and external devices such as lasers and photodetectors. TheKawachi et al. patent uses separately formed glass elements havingforty-five degree mirror surfaces for providing this function.

The value of hybrid optical packaging is that it is amenable to massproduction using known techniques of chemical vapor deposition,photolithography, and other techniques described in the Henry et al.paper. It would be desirable to be able to include turning mirrors insuch packaged devices without incurring significant additional costs.

SUMMARY OF THE INVENTION

In a structure of the type described in the aforementioned Henry et al.publication, the core of the waveguide, since it is surrounded by glassof low refractive index, is effectively embedded a small distance belowthe upper surface of the body of the glass structure. As a consequence,a turning mirror in the structure can be made by etching a cavity in theupper surface that intercepts the path of light propagated by thewaveguide. For improving reflection of intercepted light, the side ofthe cavity opposite the end of the waveguide can be metallized.Unfortunately, such a turning mirror is not very efficient because thesides of the cavity, when etched in a normal manner, do not describe anangle such as forty-five degrees, which would normally be optimum forremoving light from the end of the waveguide or injecting light into thewaveguide.

In accordance with one aspect of the invention, prior to etching thecavity, part of the interface of the mask and the body is treated suchthat the etchant undercuts the mask on the side of the cavity remotefrom the waveguide to a greater extent than it undercuts the mask on theside of the cavity adjacent the waveguide. By etching to a proper depth,and by properly treating the interface portion, one can obtain a cavitythat has a side that is nearly normal to the surface on the side of thecavity adjacent the waveguide, while having a side which is atsubstantially forty-five degrees, or some other appropriate angle, tothe surface on the side opposite the waveguide. Thus, light can beemitted from the waveguide into the cavity with minimum refraction, andcan be reflected with maximum efficiency by the side of the cavityopposite the waveguide. The angled surface is preferably metallized tooptimize such reflection. The reflective surface can also be used toinject light into the waveguide from an external source such as a laser.

In accordance with one embodiment, the treating step comprises the stepof coating part of the surface of the glass body with a material thatetches at a faster rate than the glass body. For example, a thin coatingof aluminum may be used, which can be made to have a much higher etchrate than glass. When coated over the glass body along the area that isto constitute the side of the cavity remote from the optical waveguide,the faster etching aluminum causes the side of the cavity on which it iscoated to etch at a greater angle with respect to the mask, and thisangle can be made to approximate forty-five degrees.

In another embodiment, the treating comprises the step of implantingions in part of the upper surface of the glass body. This increases therate of etch of ion implanted portion with respect to the part of thebody in which no ion implant has been made and thus makes possible theselective etch undercut desired. In another embodiment, the surface isheavily doped so as to increase its etch rate.

As will be seen from the detailed description below, empirical methodsare used to obtain a slope of the cavity which is close to forty-fivedegrees. While forty-five degrees is mentioned for illustration, otherslopes may be desired, depending upon the package structure or externaldevices with which it is used. These and other objects, features andbenefits of the invention will be better understood from a considerationof the following detailed description taken in conjunction with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of part of an optical waveguide packageillustrating one aspect of the invention;

FIGS. 2-3 and 5-6 are schematic views illustrating successive steps informing a cavity in an optical waveguide structure in accordance withone embodiment of the invention;

FIG. 4 is a view taken along lines 4--4 of FIG. 3;

FIGS. 7 and 8 illustrate a method for making a cavity in accordance withanother embodiment of the invention; and

FIG. 9 illustrates still another embodiment of the invention.

DETAILED DESCRIPTION

The drawings and description are greatly simplified in an effort todescribe clearly the nature of the invention. The drawings are not toscale, and indeed are intentionally distorted to reveal more clearlycertain features of the invention.

Referring now to FIG. 1, there is shown part of an optical waveguidestructure 10 that has been made according to the aforementioned C. H.Henry et al. paper. A silicon dioxide layer 11 is first made over asilicon substrate 12. Over the layer 11, a glass layer 13 is formed,which may have a height of five microns. The layer 13 may be of a glassknown as P-TEOS glass, which means that it is doped with phosphorous andhas been deposited by a reaction of tetraethylorthosilicate (TEOS).Layer 13 may be doped to a concentration of two percent by weight ofphosphorous so as to have a refractive index of approximately 1.4604.Upon layer 13 is formed the waveguide core layer 15, which may bephosphosilicate glass, doped with phosphorous to a concentration ofeight percent so as to have an index of refraction of approximately1.4723. After layer 15 has been formed over layer 13, it is patterned byusing masking and etching techniques to form desired waveguide paths. Asdescribed in the Henry et al. paper, the waveguide paths defined by thepatterning can cause various lightwave interactions to produce any ofvarious device functions. After patterning of layer 15, a 0.5 μm layer(not shown) of seven percent phosphorous glass may be deposited over theetched core and the entire structure heated. Heating to a temperature ofnine hundred degrees Centigrade causes the glass to reflow and the corelayer 15 to change shape from a rectangular cross-section to a moresemi-circular configuration. This smooths out any irregularities in thesurface, thus decreasing the optical loss. Next, layer 16 is depositedover it, which may be identical with layer 13. Layers 13 and 16 thenconstitute cladding layers for the optical waveguide, while layer 15 isthe core layer. Typical layer thicknesses are: layer 11, ten microns;layer 13, five microns; layer 15, five microns; layer 16, seven microns.

An optical waveguide structure typically requires a number of turningmirrors either to couple light onto the ends of waveguides or towithdraw light from waveguides. We have determined that a promisingmethod for making a turning mirror is to use masking and etching fordefining a cavity 18 in the upper surface of layer 16 adjacent the endof the waveguide defined by layer 15. That is, a polysilicon mask layer19 is formed over glass layer 16, which is selectively etched so as todefine within it an aperture 20. Thereafter, the portion of the glassexposed by aperture 20 is subjected to an etchant which attacks glass,but does not attack appreciably the mask layer 19. The etchant thereforeetches the cavity 18 into the glass body as defined by the aperture 20.Illustratively, the mask layer may be 0.5 μm polysilicon and the etchanta 7:1 buffered oxide etch with surfactant. Aperture 20 is formed byconventional photolithographic masking and etching. After cavity 18 isformed, the polysilicon mask layer 19 is removed by etching.

With the cavity etched sufficiently deep to intercept the path of thelight propagating on the optical waveguide, defined by core layer 15,the side of the cavity may be used to reflect light out of thestructure. For example, (after removal of mask layer 19) lightpropagating on the waveguide as indicated by arrow 21 would be projectedinto cavity 18 and would be reflected out of the optical waveguidepackage by the side of the cavity opposite the waveguide. The reflectingside of the cavity could be selectively metallized to improve itsreflectivity. A drawback of the structure of FIG. 1 is that thereflective surface is not necessarily at an optimum angle for retrievinglight from the package or introducing light to it. Generally speaking,it is easier to align lenses and other elements along lines nearlyperpendicular to the surface of the optical waveguide package, ratherthan at severe angles to it. This drawback is overcome by the method formaking the cavity illustrated in FIGS. 2-6.

Referring to FIG. 2, the various layers may have the same constituenciesand thicknesses as the corresponding layers of FIG. 1 and are thereforelabeled with the same reference numerals. Prior to etching the cavity, alayer 23 of a material such as aluminum having a faster etch rate thanthat of glass is deposited over the portion of the glass body intendedto constitute a reflecting surface of the etched cavity. Layer 23 may be0.5 microns thick.

Referring to FIG. 3, the mask layer 19 is deposited over the structureincluding aluminum layer 23, and an aperture 20 is formed in it asdescribed above. FIG. 4, which is a top view of the structure of FIG. 3,shows the relationship of the aperture 20 with respect to the core layer15 defining the optical waveguide and the aluminum layer 23. It ispreferred that the aperture 20 slightly overlaps the aluminum layer 23,as shown, but this aspect is not critical, as will become clear later. Acavity is etched through aperture 20 using an etchant having onecomponent that etches the aluminum layer 23 and another component thatetches the glass, but at a slower rate than the aluminum. The componentthat etches the aluminum may be a mixture of phosphoric acid, nitricacid and acetic acid, while the component that etches the glass may be7:1 buffered oxide etch with surfactant.

Referring to FIG. 5, when the etchant etches the cavity 18, aluminumlayer 23 etches at a faster rate than the glass. Under suchcircumstances, the etchant inherently attacks the glass so as to form asubstantially smooth side 25 of the cavity leading to layer 23. Anotherway of stating this is that the etchant undercuts mask 19 at a fasterrate on the side 25 of the cavity containing layer 23 than on the side26 adjacent the optical waveguide. With the normal undercut, the side 26of the cavity adjacent the waveguide is nearly normal, which isdesirable for minimizing refraction of light entering or leaving thecavity in the direction of waveguide 15. Ordinarily, the relevantparameters can be chosen such that side 25 extends at substantially aforty-five degree angle with respect to the axis of waveguide layer 15.The relevant parameters are, of course, the etch rate of the glass, theetch rate of the aluminum, the thickness of layer 23, and the depth ofthe cavity 18. These parameters are preferably arrived at empirically.Even if side 25 is not forty-five degrees, it will necessarily have alower slope than side 26 because the etch rate of layer 23 is fasterthan that of glass, and this will constitute an improvement over thestructure of FIG. 1.

Referring to FIG. 6, the mask layer is removed and the side 25 of thecavity is preferably metallized to form a metal layer 28 for enhancingreflection. Light from the waveguide propagating along path 29 will thenbe reflected out of the package as shown. With the side 26 of the cavitynearly normal with respect to the path of the lightbeam, refraction atsurface 26 will be minimized. A path having the reverse direction ofpath 29 is followed for coupling light into waveguide 15.Illustratively, the metal layer 28 may be made by sputtering over theentire surface successive layers of titanium, platinum and gold, whichare then patterned by masking and etching, all of which is known fromthe integrated circuit art. Additional gold may be electroplated if theresulting surface is smooth and specular.

Referring now to FIG. 7, another method of treating the surface of theglass to make it preferentially etch at a faster rate is to ion implanta layer 31 as shown schematically by the arrows. The boundaries of layer31 are controlled by a mask 32 which prevents ion implantation at otherportions of the glass body. It can be shown that glass layers damaged byion implantation etch at a faster rate than undamaged glass whensubjected to a glass etchant. The layer 31 may typically be 0.1 micronsthick and the ions may be argon implanted at a dosage of 1-9×10¹³ ionsper square centimeter. After implant, the mask 32 is removed, as byetching.

Referring to FIG. 8, the mask layer 19 is formed with an aperture 20which preferably overlaps slightly the layer 31. Etching throughaperture 20 forms the cavity 18 which is asymmetric for the same reasonsas described with respect to FIG. 5. Since layer 31 etches at a fasterrate than undamaged glass, it inherently causes a greater undercut ofmask 19 and consequently a controlled slope to the side 25 of the cavity18. The mask 19 is then removed, and the side 25 is metallized to formthe structure shown in FIG. 6. The etchant may be 7:1 buffered oxideetch with surfactant.

Referring to FIG. 9, a layer 33 of unannealed four percentphosphosilicate glass is formed which has a higher etch rate than thatof glass layers 13 and 16 because it is more highly doped, and thereforefunctions in the same manner as the ion implanted layer 31 of FIG. 7 andthe aluminum layer 23 of FIG. 3. Thus, when the structure is masked andetched as in FIG. 8, an asymmetric cavity is formed with the finalstructure again being that shown in FIG. 6. In our experiments, layer 33had a thickness of 0.5 micron and layers 16 and 13 were doped with twopercent phosphorus as mentioned before.

The various methods described are illustrative of methods that can beused for making turning mirrors in waveguide package bodies by etching acavity. Various methods for making the cavity asymmetric so as toproduce a desired reflective slope have been described to illustrate theconcept involved. Various materials other than those specificallydescribed may be used to give the faster etch required for asymmetricmask undercutting. For example, metals other than aluminum haveappropriately high etch rates when exposed to various etchants and wouldbe expected to work; various other glasses and other materials could beused. In general, a layer of any material may be used if an etchant canbe developed that etches the material at a higher etch rate than theglass in which the cavity is formed. The faster etch rate gives anasymmetric cavity having a reflecting side wall extending at any ofvarious angles; the optimum angle may, for example, be less thanforty-five degrees, depending on other design requirements. Variousother embodiments and modifications may be made by those skilled in theart without departing from the spirit and scope of the invention.

We claim:
 1. An optical waveguide structure comprising:a first layer ofrelatively low refractive index glass; a second layer of relatively highrefractive index glass overlying the first layer; the second layer beingpatterned to define optical waveguide portions for propagating lightalong optical paths; a third layer of relatively high refractive indexglass overlying the second layer; the third layer overlying a first endof a first waveguide portion defined by part of the second layer; and anasymmetric cavity extending through the third layer and at least part ofthe first layer which is adjacent the first end of the first opticalwaveguide portion and which intersects the optical path defined by thefirst waveguide portion, said cavity having a steeper slope on a firstside thereof which is closest to the waveguide portion than on a secondside thereof which is most remote from the waveguide portion; the secondside of the asymmetrical cavity constituting a mirror surface which iscapable of reflecting light to or from said first optical waveguideportion.
 2. The structure of claim 1 wherein:the cavity is made byforming a mask layer over the third layer; making an opening in the masklayer adjacent the first end of the first optical waveguide portion; andsubjecting at least parts of the third layer and first layer to anetchant of glass.
 3. The structure of claim 2 wherein:prior to etching,part of the interface of the mask and third layer is treated such thatthe etchant undercuts the mask on the side of the cavity remote from thewaveguide portion to a greater extent than it undercuts the mask on theside of the cavity adjacent the waveguide portion.
 4. The structure ofclaim 3 wherein:the treating step comprises the step of coating part ofthe surface of the third layer with a material that etches at a fasterrate than glass when exposed to said etchant so as to increase the rateof undercut of the mask layer.
 5. The structure of claim 4 wherein:thetreating step comprises the step of coating part of the third layer witha metal.
 6. The structure of claim 4 wherein:the treating step comprisesthe step of coating part of the third layer with a layer of glass thatis doped with a higher concentration of impurities than the third layer.7. The optical waveguide structure of claim 1 wherein:the second side ofthe asymmetrical cavity is metallized, thereby to improve thereflectivity of said mirror surface.
 8. The optical waveguide structureof claim 1 wherein:a portion of said third layer extends between saidfirst end of said first optical waveguide portion and said first side ofsaid asymmetric cavity, whereby light propagating in said firstwaveguide portion in the direction of said cavity is projected firstthrough the third layer portion, then into the cavity, and is thenreflected by said mirror surface.